1. Field of the Invention
The present invention relates to an optical information recording medium capable of recording/reproducing an information signal by irradiating a thin film formed on a substrate with a high-energy light beam such as a laser, a method for producing the medium, and a method and apparatus for recording information using the medium.
2. Description of the Related Art
A thin film made of a chalcogen material or the like formed on a substrate is irradiated with laser light to be heated locally, whereby a phase can be changed between an amorphous phase and a crystalline phase having different optical constants refractive index n; extinction coefficient k) due to the difference in irradiation conditions. This has already been well-known, and a so-called phase-change type optical information recording medium using this phenomenon has been researched and developed, and commercialized actively.
In a phase-change type optical information recording medium, an information track is irradiated with laser light, with the laser output being modulated in accordance with an information signal between at least two power levels (i.e., recording level and erasure level), whereby a new signal can be recorded while an existing signal is being erased.
In such an optical information recording medium, generally, in addition to a recording layer, a protective layer made of a dielectric material or the like having excellent heat resistance and a reflective layer made of a metal/alloy material or the like are provided. The protective layer is formed on a lower side (i.e., on a side dose to a transparent substrate that is a laser light incident side) of the recording layer, and on an upper side (i.e., on a side opposite to the transparent substrate) of the recording layer, for the purpose of preventing evaporation of the recording layer and thermal deformation of the substrate during repeated recording, and enhancing the light absorbance and optical change of the recording layer by an optical interference effect, etc. The reflective layer is formed on a side opposite to the transparent substrate with respect to the recording layer, for the purpose of using incident light efficiently, and increasing a cooling speed so as to obtain an amorphous phase easily, etc.
To provide an interface layer between a recording layer and a dielectric layer is proposed (e.g., see JP 5(1993)-217211 A and WO 97/34298). It is desirable that the interface layer has functions of promoting crystallization of a recording layer to enhance erasure characteristics, preventing interdiffusion of atoms and molecules between a recording layer and a dielectric protective layer to enhance durability during repeated recording, and the like. The interface layer also is desired to have environmental reliability that prevents peeling and corrosion between the recording layer and the dielectric layer
Furthermore, to provide a material layer having a high refractive index, absorbing light appropriately, between an upper dielectric layer and a reflective layer also is proposed (e.g., see JP 2000-215516 A). The purpose of providing such a material layer is to adjust the ratio between a light absorbance in the case where a recording layer is crystalline and a light absorbance in the case where the recording layer is amorphous, so as not to allow a mark shape to be deformed during overwriting. This enhances an erasure ratio, and increases the difference between reflectance in the case where the recording layer is crystalline and reflectance in the case where the recording layer is amorphous to increase a C/N ratio, etc.
As a basic means for increasing the amount of information to be accumulated in one such optical information recording medium, there is a method for shortening the wavelength of laser light or increasing the numerical aperture of an objective lens for collecting the laser light, thereby decreasing a spot size of the laser light to enhance a recording surface density. It is the recent tendency to use an optical system with a wavelength of 660 nm and a numerical aperture of about 0.6, as in a recording digital versatile disk (DVD). Furthermore, the following also is considered: a blue laser diode emitting light with a wavelength in the vicinity of 400 nm, which is approaching a practical stage, is used to enhance the numerical aperture to about 0.85. When the numerical aperture is enhanced in such a manner, the allowable width of the optical information recording medium with respect to a tilt is decreased. Therefore, to decrease the thickness of a transparent. substrate on a laser light incident side from 0.6 mm of a recording DVD to about 0.1 mm also is proposed.
A recording medium having a multi-layered configuration (hereinafter, referred to as a “multi-layered recording medium”), in which a plurality of layers for recording/reproducing information are stacked so as to increase the amount of information to be handled by one medium, also is proposed. In such a multi-layered recording medium, an information layer on a side close to a laser light source absorbs light, so that an information layer on a side away from the laser light source is recorded/reproduced with attenuated laser light, which causes a decrease in sensitivity during recording and a decrease in reflectance/amplitude during reproducing. Thus, in the multi-layered recording medium, it is necessary to increase the transmittance of the information layer on a side close to the laser light source and to increase the reflectance, reflectance difference, and sensitivity of the information layer on a side away from the laser light source, thereby enabling sufficient recording and reproducing characteristics to be obtained with a limited laser power.
In the optical information recording medium, it is important to enhance a recording density as described above. It also is important to increase a recording speed so as to handle a great amount of data in a short period of time. In order to handle high-speed recording, it is necessary to increase a crystallization speed of a recording layer. In a typical recording material Ge—Sb—Te, in particular, a composition in the vicinity of GeTe—Sb2Te3 (a composition in the vicinity of GeTe—Sb2Te3 line in the triangular phase diagram of Ge, Sb, and Te), it is known that Sn is substituted for a part of Ge, whereby a crystallization speed is increased.
As described above, the recording speed of a newly developed recording and reproducing apparatus tends to increase, and there is a demand for a medium that can handle such the tendency. Simultaneously, in order to keep compatibility with respect to an existing drive that performs recording only at a low speed, it is necessary that the same medium also can be recorded at a low speed. In order for a medium to handle high-speed recording, it is necessary to use a recording layer with a high crystallization speed as described above. When such a recording layer is used for recording at a low speed, a crystallization speed is too high. More specifically, an amorphous phase is unlikely to be formed, and a mark is unlikely to be large, which decreases the signal amplitude. In particular, in the case where Sn is substituted for a part of Ge as described above, a change in an optical constant between a crystalline phase and an amorphous phase is decreased, which may promote the decrease in a signal amplitude.